Influence of the jaw tracking technique on the dose calculation accuracy of small field VMAT plans

Purpose The aim of this study was to evaluate experimentally the accuracy of the dose calculation algorithm AcurosXB in small field highly modulated Volumetric Modulated Arc Therapy (VMAT). Method The 1000SRS detector array inserted in the rotational Octavius 4D phantom (PTW) was used for 3D dose verification of VMAT treatments characterized by small to very small targets. Clinical treatment plans (n = 28) were recalculated on the phantom CT data set in the Eclipse TPS. All measurements were done on a Varian TrueBeamSTx, which can provide the jaw tracking technique (JTT). The effect of disabling the JTT, thereby fixing the jaws at static field size of 3 × 3 cm2 and applying the MLC to shape the smallest apertures, was investigated for static fields between 0.5 × 0.5−3 × 3 cm2 and for seven VMAT patients with small brain metastases. The dose calculation accuracy has been evaluated by comparing the measured and calculated dose outputs and dose distributions. The dosimetric agreement has been presented by a local gamma evaluation criterion of 2%/2 mm. Results Regarding the clinical plans, the mean ± SD of the volumetric gamma evaluation scores considering the dose levels for evaluation of 10%, 50%, 80% and 95% are (96.0 ± 6.9)%, (95.2 ± 6.8)%, (86.7 ± 14.8)% and (56.3 ± 42.3)% respectively. For the smallest field VMAT treatments, discrepancies between calculated and measured doses up to 16% are obtained. The difference between the 1000SRS central chamber measurements compared to the calculated dose outputs for static fields 3 × 3, 2 × 2, 1 × 1 and 0.5 × 0.5 cm2 collimated with MLC whereby jaws are fixed at 3 × 3 cm2 and for static fields shaped with the collimator jaws only (MLC retracted), is on average respectively, 0.2%, 0.8%, 6.8%, 5.7% (6 MV) and 0.1%, 1.3%, 11.7%, 21.6% (10 MV). For the seven brain mets patients was found that the smaller the target volumes, the higher the improvement in agreement between measured and calculated doses after disabling the JTT. Conclusion Fixing the jaws at 3 × 3 cm2 and using the MLC with high positional accuracy to shape the smallest apertures in contrast to the JTT is currently found to be the most accurate treatment technique.


| INTRODUCTION
The use of small radiation fields in radiotherapy has increased substantially, in particular, in treatments with stereotactic beams and non-uniform fields that are composed of multiple small subfields like in volumetric modulated arc therapy (VMAT), which has become the treatment of choice for an increasing number of treatment sites. [1][2][3][4][5] This state-of-the-art irradiation technique for the delivery of highly conformal radiation fields to the target volume requires complex dose calculation algorithms in the treatment planning system (TPS) as well as sophisticated medical linear accelerators (linacs). The rationale behind this increasing use of VMAT in stereotactic radiotherapy treatments is the possibility of improved healthy tissue sparing, the ability to create very steep dose gradients and the reduction in treatment time compared to intensity modulated radiation therapy (IMRT) and conventional treatments. 4,5 To exploit fully the benefits of stereotactic treatments, high spatial accuracy in dose delivery is vital. In stereotactic treatments, spatial and dosimetric accuracy are inextricably linked which makes the evaluation of the dosimetric accuracy of small field delivery equally important. The IPEM report no 103 6 summarizes many studies discussing the difficulties of small field dosimetry and modeling. Accuracy could be limited due to the characteristics of the detectors used for measurements or due to approximations of Monte Carlo simulations used to model narrow beam doses in the TPS. 7,8 VMAT plans are typically highly modulated and a considerable fraction of the total control points used for a plan consists of small subfields. 9 When evaluating VMAT plan accuracy, three aspects should be considered: the dosimetric measurements required for TPS commissioning, the beam model in the TPS, and the accuracy of verification measurements. The first and third aspect include the dosimetry of small fields, which is challenging due to the lack of charged particle equilibrium (CPE), partial blocking of the beam source giving rise to pronounced and overlapping penumbra and the availability of small detectors for sizes comparable to field dimensions. 10 CPE is associated with the range of secondary particles and thus dependent on the beam energy and the density of the medium. The choice of radiation detectors in small fields is crucial as they usually perturb the secondary electron fluence due to its presence and composition. The second aspect influencing VMAT plan accuracy refers to properly designed multisource modeling using accurate measured data together with accurate dose calculation algorithms that handle non-CPE conditions to provide acceptable dose distribution. It is important to realize the details of the improvements and limitations in the source model of the TPS and to know the inaccuracies of the input data which are used to optimize the source model parameters. The input data used for beam configuration which have an impact on the dose calculation accuracy are handled in section 2.B.

Several planning and measurement studies on clinical stereotactic
VMAT plans for small target volumes have been published. For instance, Lagerwaard et al. 11 and Verbakel et al. 12 have carried out film measurements of VMAT plans for a range of tumor sizes, resulting in good agreement between the calculated and measured dose distributions. Remark that in these studies the convolution-based anisotropic analytical algorithm (AAA) has been used, while this work evaluates the AcurosXB calculation algorithm which belongs to the class of the Linear Boltzman Transport Equation solvers, allowingsimilarly to the Monte Carlo-based methodsaccurate modeling in heterogeneous media. 13,14 The accuracy of these two photon dose calculation algorithms (AAA, AcurosXB) compared to 2D and point measurements for small fields usable in stereotactic treatments has been investigated in Fogliata et al. 13 and found to be acceptable using adequately tuned configuration parameters in the TPS, originally developed for standard external beam therapy for broad beams from conventional linear accelerators. Conversely, the work of Fog et al. 9 cautioned about the use of small fields with VMAT and they reported large discrepancies up to 53% between measured and calculated doses for static fields one MLC leaf wide. For a small field VMAT plan with a 0.4 cm 3 planning target volume about 10% overdosing was detected (Eclipse version 8.6, 2.5 mm grid spacing) and more modulation in the plan was measured than calculated. 9 Since the investigation of Fog et al. 9 has been performed using an older version of the TPS, the purpose of this work is to examine small field VMAT plan accuracy more thoroughly with a recent version of TPS and a new generation of linac.
Furthermore, the jaw tracking technique (JTT) provided by the latest types of linacs keeps jaws during dose delivery as close as possible to the MLC aperture, thereby minimizing leakage and transmission through the MLC leaves resulting in optimized organs-at-risk (OAR) sparing and potentially improving the dose falloff towards the surrounding critical structures. 15 We aim to investigate the impact of disabling the JTT on the dose calculation accuracy to the target, for a range of small static fields as well as for a number of stereotactic radiosurgery treatments with very small target sizes. The suggestion to keep the jaw settings above a minimum size and to generate shielding and modulation by the MLC only, already appeared in the literature, 13,16 but to our knowledge has never been investigated thoroughly using clinical treatment plans for small treatment volumes.

| METHODS
To evaluate experimentally the accuracy of the dose calculation algorithm in the TPS in small field highly modulated VMAT treatments, 28 clinical VMAT treatment plans characterized by small target SWINNEN ET AL.
| 187 volumes and fraction doses between 2.75-30 Gy were delivered on a commercial high resolution 2D detector array in a rotational phantom for the independent 3D validation of the calculated dose distribution. Also, the influence of disabling the JTT on the dose calculation accuracy to the target is investigated for a range of small static fields as well as for a number of stereotactic radiosurgery treatments with very small target sizes. To validate the 2D detector array measurements, the small static fields were also delivered to radiochromic film (see Appendix B).

2.A | Measurement system for 3D dose verification
The Octavius 4D system consists of an Octavius 4D phantom, a 2D detector array and a VeriSoft software package (version 6.2) for data collection and analysis (PTW-Freiburg). Controlled by an inclinometer, the Octavius 4D phantom rotates synchronously with the gantry, taking time-and gantry angle-resolved dose measurements. 17 The detector array used in this work is the Octavius 1000SRS model which consists of 977 liquid-filled ionization chambers covering an area of 11 9 11 cm 2 . Each detector covers a cross section of 2.3 9 2.3 mm 2 with a height of 0.5 mm, resulting in an active volume of approximately 0.003 cm 3 . In the inner 5.5 9 5.5 cm 2 area of the array, the centers of adjacent chambers are placed at a distance of 2.5 mm from each other. 18 The detector size and the center-tocenter distance of the detectors are important parameters for accurate spatial measurement of complex dose distributions with steep dose gradients. In Poppe et al. 18

2.B | Treatment planning
Treatment planning was carried out with the Eclipse TPS (Varian Medical Systems Inc., Palo Alto, CA, USA) version 11, using the AcurosXB photon dose calculation algorithm version 10.0.28. A calculation grid spacing of 2.5 mm is the default setting and is generally applied for clinical calculations as smaller grid spacing may require impractically large computation times. In addition, the minimum available grid spacing of 1 mm has been used for very small targets.
The Eclipse TPS was originally designed for 3D conventional broad photon beam radiotherapy treatments. Details of the most important incremental improvements from the point of view of accurate dose calculation for small field sizes are presented in Torsti et al. 16 Currently, there are still limitations in the source model. 19 It is important to remember that the head of the linac is not physically modeled. The complex interactions from all different components of the head can only be approximated: the phase space of the head scatter source is approximated as a planar distributed source with a Gaussian shape located at the bottom of the flattening filter. A second important source of scattered radiation in the head is located at the edge of the primary collimator. Furthermore, the source model parameters are optimized using symmetric jaw defined beam data with MLC retracted. It is a shortcoming that no asymmetric nor MLC-delimited fields can be put into the configuration program. In our Eclipse Beam Configuration workspace, the imported beam data are measured relative profiles, depth dose curves and output factors (OF) for field sizes between 1 9 1 cm 2 to 40 9 40 cm 2 (respectively the smallest and largest possible field size fields as input, defined by jaws only) while the more concise set recommended by Varian contains field sizes defined by jaws between 3 9 3 cm 2 and 40 9 40 cm 2 . As described in the Varian white paper, 16 the relative profiles and depth dose curves below 3 9 3 cm 2 can be imported in the configuration workspace but are not needed and smaller than 2 9 2 cm 2 even not used in the configuration program, while the OF below 3 9 3 cm 2 are used. How these OF for fields < 3 9 3 cm 2 are applied in the TPS is discussed in section 4. This means that the OF for small fields have to be measured very accurate, which is also stressed in the IPEM report no 103. 6 An ideal detector for small field output factor measurements would be the one having a uniform spectral response, a high signal-to-noise ratio whilst being smaller than half the size of the region which can be considered acceptably uniform, and water equivalent. Unfortunately, there is no commercially available detector which can accurately measure OF of small radiation fields without requiring corrections for volume averaging or non-water-equivalent dosimetric properties. Therefore, it is 'good practice' to compare several different detector types. 6 The use of a variety of detectors when measuring output factors for small fields helps to reduce the uncertainty in the estimation of the true value. In our case, we measured the OF between The small field OF from 3 9 3 cm 2 down to 1 9 1 cm 2 led to comparable measurement results for the investigated dosimeters (within 0.6% and 1.3% agreement for respectively 6 MV and 10 MV photons). In the Varian Beam Configuration Reference Guide is recommended that you do not need to measure all OF, just type in some values and interpolate the rest. However, we applied our own interpolation procedure using a selfmade matlab script leading to a less coarse result: for 1 9 1 cm 2 the OF for 6 MV and 10 MV photon beams is 0.624 and 0.706 respectively, compared to the average measurement value of 0.650 and 0.712, which gives a deviation of 5% and 0.9%. From these findings, we estimate an inaccuracy on the OF measurements for 1 9 1 cm 2 of maximum 5%.
The AcurosXB algorithm uses the multiple source model 19  Clinical patients (n = 28) were chosen with various lesion sites: lung, brain, prostate and spine. All were characterized by rather small to very small target volumes (PTV between 96.5 and 0.54 cm 3 ) and fraction doses between 2.75-30 Gy. The clinical treatment plans were projected on the artificial homogeneous phantom CT data set provided with the Octavius 4D system and recalculated in the TPS.
In the TPS, the electron density of the Octavius 4D phantom relative to water has been set to 1.016 according to the manufacturer's recommendation.

2.C | Treatment technique
The treatment technique used was VMAT with RapidArc â (RA) (Varian Medical Systems), which is based on an inverse planning method to deliver an intensity modulated dose distribution using a MLC of which the movement is modulated to the target volume while the beam is on and the gantry moves around the patient. This MLC movement can be very complex, containing many small field segments.

2.D | Treatment delivery
All data were gathered in 6 MV and 10 MV photon beams. By placing the 1000SRS array's active layer in the isocenter, the distances between the ionization chambers in the inner part of the array are thus equal to the projected leaf widths of the HD 120-MLC at the isocenter. The effect of disabling jaw tracking, thereby fixing the collimator jaws at 3 9 3 cm 2 and applying the MLC to shape the small- Prior to the measurements, the jaw position calibration has been verified using radiochromic EBT3 film (Ashland Inc., Covington, KY, USA) and the reproducibility of jaw position calibration with 0.1 mm can be confirmed by the regular quality control procedures following NCS report eight guidelines. 20

2.E | Octavius 4D analysis method
To evaluate the dosimetric agreement between the 1000SRS mea-

3.A | VMAT treatment plan verifications
The errors in the device set-up reproducibility were minimized by making cone beam computed tomography (CBCT) images of the setup before each set of measurements and using the online matching procedure for shifting the corresponding CBCT with the reference planning CT scan. Regarding the VMAT treatment fields, mean AE SD of the volumetric gamma evaluation scores for the dose difference and distance to agreement criteria of 2% and 2 mm with 10%, 50%, 80% and 95% cut-off dose values were 96.0% AE 6.9%, 95.2% AE 6.8%, 86.7% AE 14.8% and 56.3% AE 42.3% (n = 28), respectively. An average pass rate of 96.0% with quite a large spread  (Table I) are not affected by the OF down to 1 9 1 cm 2 used for beam configuration. For field sizes < 2 9 2 cm 2 , the quantity |D (Oct4D,TPS)jaws ÀD (Oct4D,TPS)MLC |(%) ( Table I) represents an improvement in dose calculation accuracy that can be achieved by fixing the jaws at 3 9 3 cm 2 and letting the MLC form the smallest apertures.
The gamma agreement scores and comparison of measured versus calculated maximum doses for seven clinical patients with small brain lesions are shown in Table II. The patients are ordered in such a way that the smallest field size area belongs to patient 1 and the largest to patient 7. Re-planning of the seven patients with disabling of the JTT and fixing the collimator jaws at 3 9 3 cm 2 leads to a substantial improvement in the agreement scores for the higher dose regions in Table II. From Table II can also be seen that there is an improvement in correspondence between measured and calculated maximum doses after fixing the collimator jaws at 3 9 3 cm 2 . In general, it can be noticed that the improvement is increasing with decreasing field size.

3.B.2 | EBT3 film
The results on the comparison between EBT3 film and Octavius 4D measurements can be found in Appendix B.

| DISCUSSION
In this work, the main purpose was to quantify the accuracy of the dose calculation algorithm for clinical small field VMAT plans.
The user of the TPS has little influence on the number of subfields to be included in the planas there is no parameter which controls the smoothness of dose modulationnor has the user information on the smallest field size for which dose computation has acceptable accuracy. The only ability the user has to minimize the errors arising from unnecessary use of very small subfields in RA plans is to use a MU constraint. As a rule of thumb, we accept VMAT treatment plans with a maximum number of MU equal to 3 times the prescription dose (in cGy). 62% of the VMAT plans in this study have 10% or more of their fields smaller than 2 cm 2 . So 62% of the VMAT plans are directly affected by the uncertainties in small F I G . 1. Plot of the relative differences in maximum dose as a function of field size area defined by collimator jaws for VMAT treatment plans. (D max is referring to the maximum dose in the 3D volume; Patients A and B are further discussed in the section 3.A).
field dosimetry. The Octavius 4D measurements of the VMAT treatment fields suggest that the use of very small subfields in small field VMAT plans may be the cause of the high discrepancies between the calculated and measured doses (Fig. 1).
A possible explanation to the trend observed in Fig. 1 can be that Eclipse was originally designed for 3D conformal radiotherapy and thateven after several important and effective improvements    T A B L E I I Measured versus calculated maximum doses with jaw tracking on (white rows) against fixed collimator jaws at 3 9 3 cm 2 (shaded rows) for seven brain metastases patients.  WCF is a wedge correction factor for hard field wedges and not applicable in our case (WCF = 1).
Obviously, the observed CBSF table shape (Fig. 2) looks quite unphysical compared to the measured physical collimator backscatter factors at small and large field sizes. Of course we would expect the CBSF to be highest for the smallest field size, namely at 1 9 1 cm 2 , but this "jump" in the curve cannot be due to inaccuracies in the measurement of output factor at 1 9 1 cm 2 alone. The differences between the physical back scatter factors and the configured values reveal information about the remaining limitations of the source model and are due to the fact that the CBSF in the source model beam data is a residual correction factor taking into account all phenomena of phantom scatter and head scatter that are not otherwise accounted for by the source model or the dose deposition engine. 16 Finally, a shortcoming in the Eclipse TPS version 11 is the fact that TrueBeam cannot be selected as Machine Type in the Parameter View of the Beam Configuration program. Varian advises the customers to use a C-series instead although some difference in the backscatter factors between C-Series and TrueBeam machines can be expected. 24 Recently published papers about CBSF show a large difference between the CBSF in Torsti et al. 16 and the ones published by Sibolt et al. 25 and Zavgorodni et al. 24 Sibolt et al. 25  Output factors for fields smaller than 3 9 3 cm 2 , on the other hand, should be measured very accurately with a reliable detector, since these are applied in the dose calculation. Up to now, Varian does not recommend the use of output factors for very small fields in the configuration process. 13 Differences up to 5% between measurements and calculations of small field output for 6 MV photon beams were found for configurations including the 3 9 3 cm 2 as minimum field size, 13 which is of the same order what we found in D (Oct4D,TPS) MLC (%) for 6 MV photons in Table I. For 10 MV photon beams, this factor in Table I shows even larger differences. We found that fixed jaws for field sizes smaller than 3 9 3 cm 2 (FS MLC in Table I) leads to better agreement between measured and calculated maximum doses, as these data are not affected by the OF down to 1 9 1 cm 2 used for beam configuration. Table II illustrates that the smaller the field size, the larger the impact of jaw fixation at 3 9 3 cm 2 on the dose difference results: the improvement in agreement between measured and calculated doses is the highest for the smallest target volume. Therefore, as already suggested in the literature 13,16 but never reported for clinical treatment plans, disabling jaw tracking is an option to increase the dose prediction accuracy by Eclipse for very small target volumes. Moving the jaws away from the leaf ends, typically 1 to 2 cm, increases the output and penumbra width for small fields because of more photons leaking through the leafs. 26 From Table II we believe that the TPS models this jaw effect on the small MLC fields quite well. Retracting the jaws to 3 9 3 cm 2 can lead to higher OAR doses, but AcurosXB predicts the dose under small MLC defined field segments well. 27 Focusing on the gamma agreement scores for the lower cut-off dose regions in Table II, it can be seen that the MLC leakage is well modeled by the TPS.
Moreover, out-of-field doses in regions shielded by the MLC (or both the MLC and the jaws) in RA plans were studied in Fogliata in Table I and |D (Oct4D,EBT3)jaws ÀD (Oct4D,EBT3)MLC |(%) in Table III, can be achieved by fixing the jaws at 3 9 3 cm 2 for the field sizes smaller than 3 9 3 cm 2 and letting the MLC shape the smallest apertures. However, the 1000SRS measurements for the 0.5 9 0.5 cm 2 fields ( Fig. 3(a)) show less deviation with TPS than the ones by film, which can be explained by the fact that this field size is probably too small for correct measurement by the 1000SRS. In this situation, the focal spot (source) will be almost fully occluded by the collimator jaws or MLC as seen from the isocenter. For the latter 2D comparison, the spatial resolution of EBT3 film is superior to the 1000SRS.  To conclude, our recommendations to minimize the effects of calculation uncertainties in RA plans for very small target sizes: • A calculation grid size of 1 mm is found to be superior to 2.5 mm.
• An upper MU constraint should be assigned during optimization to avoid plans with high modulation and very high number of MUs increasing treatment time.
• Fixed collimator jaws at 3 9 3 cm 2 should be used for small target volumes where the JTT leads to jaw defined field sizes smaller than 3 9 3 cm 2 .

CONFLICT OF INTEREST
The authors declare no conflict of interest.

APPENDIX B
To Further recommendations mentioned in Mathot et al. 30 were followed. Due to the anisotropic light scattering in radiochromic films, film orientation must be kept constant, which is in this study "landscape" (when the long axis of the film is perpendicular to the scanner lamp). Furthermore, the reproducibility of film positioning is an important issue due to the non-uniform scanner response over the scan field perpendicular to the scan direction.
The well-known "lateral response effect" causes transmission pixel values to decrease as the lateral distance from the scan axis increases. Therefore, we used a ruler template on the scanner glass so that film pieces can be placed in the central scan axis of the scanner bed. Due to the small field size and accurate positioning of the film in the center of the scanner, lateral response artefacts, turned out to be negligible. 31 The comparison between the EBT3 film with Octavius 4D measurements for open squared fields 2 9 2, 1 9 1 and 0.  Table III. Factors D (Oct4D,EBT3)jaws and D (Oct4D,EBT3)MLC (%) in Table III and Figs. 3(b) and 3(c) show a good agreement between both detector methods, except for 0.5 9 0.5 cm 2 fields (Fig. 3(a)), where the resolution of the 1000SRS is limiting.